Last Updated Mar 26, 2015 7:25 AM EDT
SEYNE-LES-ALPES, France -- The cockpit voice recorder recovered from the wreckage of Germanwings Flight 9525 indicates that one of the pilots was locked out of the cockpit before the plane crashed, The New York Times reported Wednesday evening.
The newspaper said a senior French military official involved in the investigation had told them that on the recording, one of the pilots is heard leaving the cockpit, then banging on the door with increasing urgency in an unsuccessful attempt to get back in.
"The guy outside is knocking lightly on the door and there is no answer," the source told the Times. "And then he hits the door stronger and no answer. There is never an answer."
Eventually, the newspaper quotes the official as saying, "You can hear he is trying to smash the door down."
The Associated Press reported early Thursday that an official with knowledge of the recordings told it one of the pilots apparently was locked out of the cockpit when the plane went down. The AP said the official was not authorized to speak publicly about the investigation.
U.S. officials contacted by CBS News could not confirm the Times or AP reports.
The Times' source, whom the newspaper said could not be identified because the probe was ongoing, said officials didn't know why the pilot left the cockpit. He also did not speculate on why the other pilot didn't open the door or make contact with ground control before the crash.
Either way, if proven accurate, CBS News correspondent Mark Phillips says the reported details from the cockpit voice recorder will change the nature of this crash from an accident, into something else.
A video produced by the Airbus, maker of the A320 passenger jet used on Germanwings Flight 9525, shows how the cockpit security system was designed in 2002, after the Sept. 11 terror attacks. It shows that flight crew members in the cabin can access the cockpit with an code to open the door, but it doesn't deal with what changes might have been made since, and it doesn't deal with the possibility of what would happen if one of the pilots deliberately tries to lock out the other.
On the Airbus, like virtually every other commercial passenger jet since 9/11, the pilot or whomever has control of the cockpit has the ultimate override power to prevent others from entering from the plane's cabin.
Martin Riecken, Lufthansa's director of corporate communications for Europe, told CBS News Thursday, "We have not received any information from the authorities leading the investigation that would support the things stated in the New York Times article. We are working on obtaining more information but will not participate in any kind of speculation. The investigation on what caused the accident falls to the responsible authorities."
CBS News aviation and transportation safety analyst Mark Rosenker, a former chairman of the National Transportation Safety Board, said, "I am concerned about a leak like this without an individual from authority specifically stating -- and I suspect the questions will be coming fast and furious to the (French accident investigation bureau) BEA to say, 'Wait a second: Is this the truth or is this something that is just kind of speculation?"'
"I am very concerned that the BEA has not responded immediately to this allegation" Rosenker said.
If true, the account in the Times report would be "absolutely extraordinary," Rosenker said. "It's difficult for me to get my head around this, given the fact that, typically, when one of the pilots leaves the cockpit, a flight attendant normally would come in, just for the ability to re-open the door. And particularly on an (Airbus) A320, which has a pretty sizable flight deck. It's not something that you could lean over the seat and flip the switch to re-open the door. You would pretty much have to get out and literally walk a couple of feet to get to that door.
"So I am kind of totally confused on how this could happen if in fact they were following the protocol or if in fact they didn't have a policy. Either way, this is still very confusing to me."
In Europe, there is no obligation in Europe for a second member of crew to take the place of pilot who leaves the cockpit, according to Gil Roy, editor of aviation news website Aerobuzz and himself a private pilot.
He tells CBS News there is a rule in Europe that both pilots cannot leave cockpit at same time -- cannot leave it empty. All European carriers' cockpit doors are locked and reinforced, he says.
Since the Sept. 11 terrorist attacks, airlines in the U.S. don't leave one pilot alone in the cockpit. The standard operating procedure is that if one of the pilots leaves -- for example, to use the bathroom -- a flight attendant takes his or her spot in the cockpit.
Investigators have not released the names of either pilot. Lufthansa CEO Carsten Spohr, himself a pilot, said he found the crash of a plane piloted by two experienced captains "inexplicable."
The first half of Germanwings Flight 9525 was chilling in its normalcy. It took off from Barcelona en route to Duesseldorf, climbing up over the Mediterranean and turning over France. The last communication was a routine request to continue on its route.
Minutes later, at 10:30 a.m. Tuesday, the Airbus A320 inexplicably began to descend. Within 10 minutes it had plunged from its cruising altitude of 38,000 feet to just over 6,000 feet and slammed into a remote mountainside.
To find out why, investigators have been analyzing the mangled "black box" that contains an audio recording from the cockpit.
Remi Jouty, the head of the BEA, said Wednesday that it has yielded "sounds and voices," but so far not the "slightest explanation" of why the plane crashed, killing all 150 on board.
Rosenker observed, "Right now, it appears that the cockpit voice recorder would be providing us with more information than, potentially, the flight data recorder if (the Times report) is true, in that an individual did leave the flight deck and (was locked out). We don't know if it was the captain. We don't know if it was the first officer."
French officials gave no details from the recording on Wednesday, insisting the cause of the crash remained a mystery. They said the descent was gradual enough to suggest the plane was under the control of its navigators.
"At this point, there is no explanation," Jouty said. "One doesn't imagine that the pilot consciously sends his plane into a mountain."
Jouty said "sounds and voices" were registered on the digital audio file recovered from the first black box. But he did not divulge the contents, insisting days or weeks will be needed to decipher them.
"There's work of understanding voices, sounds, alarms, attribution of different voices," the BEA chief said.
Confusion surrounded the fate of the second black box. French President Francois Hollande said the casing of the flight data recorder had been found in the scattered debris, but it was missing the memory card that captures 25 hours' worth of information on the position and condition of almost every major part in a plane. Jouty refused to confirm the discovery.
French officials said terrorism appeared unlikely and Germany's top security official said there was no evidence of foul play.
As authorities struggled to unravel the puzzle, Hollande, German Chancellor Angela Merkel and Spanish Prime Minister Mariano Rajoy converged on the remote accident site to pay their respects to the dead -- mostly German and Spanish citizens among at least 17 nationalities.
"This is a true tragedy, and the visit here has shown us that," Merkel said after she and Hollande overflew the desolate craggy mountainside.
Helicopters ferried in rescue workers and other personnel throughout the day. More than 600 rescue and security workers and aviation investigators were on site, French officials said.
Germanwings CEO Thomas Winkelmann said the airline was in the process of contacting victims' families. He said the 144 passengers and six crew members included 72 Germans, 35 Spaniards, three Americans and two people each from Australia, Argentina, Iran, Venezuela, and one person each from Britain, the Netherlands, Colombia, Mexico, Japan, Denmark, Belgium and Israel.
The three Americans included a mother and daughter, the U.S. State Department said. Some of the victims may have had dual nationalities; Spain's government said 51 citizens had died in the crash.
Two babies, two opera singers and 16 German high school students and their teachers returning from an exchange program in Spain were among those who lost their lives.
The principal of Joseph Koenig High School, Ulrich Wessel, called the loss a "tragedy that renders one speechless."
In Spain, flags flew at half-staff on government buildings and a minute of silence was held in government offices across the country. Parliament canceled its Wednesday session.
Barcelona's Liceu opera house held two minutes of silence at noon to honor the two German opera singers, Oleg Bryjak and Maria Radner, who were returning home after a weekend performance at the theater.
Germanwings canceled several flights Wednesday because some crews declared themselves unfit to fly after losing colleagues.